This invention relates generally to machines for rolling and curving metal sheets, and more particularly to a machine adapted to impart a desired compound curvature to a printing plate whereby the shaped plate is conformable to the surface of a printing cylinder.
In the flexographic process, printing is effected by rubber printing plates mounted on printing cylinders, the paper to be printed being impressed on the inked printing plates. In lieu of mounting rubber plates directly onto the surface of the printing cylinder, in recent years it has become the common practice to make use of metal-backed rubber plates which are attachable to the printing cylinder.
Metal-backed rubber plates have several significant advantages over plain or unbacked rubber plates. Because of the restraining action of the metal backing during the rubber molding process, improved color-to-color registration is realized. And because the register is molded into the metal-backed plate, it can be positioned in only one way, making it easier for the pressman to attach the plate to the printing cylinder.
Moreover, by punching holes in the metal-backed rubber plate in the non-printing areas thereof, one is able to use dowel pin registering devices whereby accurately-placed pins installed on the printing cylinder cooperate with these holes. It also becomes feasible by using a metal backing of ferromagnetic material, to securely attach the plate to a cylinder having magnetic elements embedded therein, thereby obviating the need for mechanical holding devices such as straps, clamps and bands.
The use of metal-backed printing plates is not limited to the flexographic process, for some dry-offset and letter press plates are provided with metal backings of sheet aluminum, printing being effected by means of an etched zinc or magnesium face layer.
All of the above-described printing plates are fabricated in the flat or planar state, and it becomes necessary therefore that these plates have a curvature set therein so as to conform them to the circumference of the printing cylinder onto which they are to be mounted, provided that these plates are to be held or clamped to the cylinder only along their circumferential edges and not by their ends.
The most commonly used machines for imparting a desired curvature to a flat printing plate are of the two-roll type, such as those disclosed in U.S. Pat. Nos. 3,304,757 and 3,371,513. In these machines, a forming roller of hard metal cooperates with a driving roller of urethane to define a nip for receiving the plate to be curved The deflectable urethane driving roller presses against the printing plate and the working surface of the driving roller acts to wrap the plate against the metal forming roller under high pressure, thereby curving the plate to the desired radius in a single pass.
The main problem encountered with all printing plates that are etched, molded or otherwise relieved to generate a printing surface, whether the plates are formed of metal-backed rubber or are formed entirely of metal or of some other material, is that the neutral plane about which bending actually takes place, does not remain in the same theoretical plane throughout the curving process but shifts position. The reason for this shift is that the thickness of the plate varies in a random manner since some areas of the plate are relieved and others are not, depending on the print pattern. Typically, the height of the printing surface above the non-printing surface is about 0.030, "whereas the overall thickness might be 0.125".
The extent to which there is a shift in the neutral plane at any one section of the plate depends, of course, on the depth of relief--the greater the depth, the more pronounced the shift. Inasmuch as the radius of curvature of the plate is a direct function of the location of the neutral plane, with known types of two-roll curving machines, this presents a problem, for it becomes difficult if not impossible to impart a uniform or predictable curvature to the plate.
All curving machines of the type heretofore known, including the three-roll benders used in the metal working industry as well as the two-roll type disclosed in the above-identified patents, because of the above-noted shift in the neutral plane, tend to develop flat segments on relief-printing plates during the curving process. These flat segments only approximate an arcuate shape and give rise to poor quality printing.
Another serious problem experienced with a two-roll curving machine, whether used with a metal plate or a metal-backed rubber printing plate, is that an enormous amount of pressure is required to curve metal which is tempered and extremely tough. When the necessary high pressure is developed at the bite between abutting rollers, one of the rollers tends to deflect and belly out and this belly is imparted to the sheet metal being rolled. Though U.S. Pat. No. 3,371,513 seeks to avoid bellying by the use of a forming roller having antideflection characteristics, the use of an anti-deflection forming roller, because of its construction, imposes certain minimum diameter requirements and this, in turn, limits the minimum radius of curvature that can be imparted to the plate being worked on.
A particular requirement which existing curving machines cannot fulfill is with regard to metal-backed flexographic printing plates. With such plates, it is desirable that the radius of curvature be slightly larger at the edges of the plate than at the middle, the transition or increment of radius increasing gradually from the center of the plate outwardly toward either edge, thereby generating a hyperbole of revolution. Hence a properly curved, metal-backed flexographic printing plate should have a compound curvature.